LTC3890EGN-1 LINER [Linear Technology], LTC3890EGN-1 Datasheet - Page 17

• Manufacturer Part Number: LTC3890EGN-1
• Description: 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
The sense resistor values are:
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
Ensure that R1 has a power rating higher than this value.
If high effi ciency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor, due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher effi ciency at heavy loads.
Peak effi ciency is about the same with either method.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. So why would anyone
ever choose to operate at lower frequencies with larger
components? The answer is effi ciency. A higher frequency
generally results in lower effi ciency because of MOSFET
switching and gate charge losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current, ΔI
tance or higher frequency and increases with higher V
APPLICATIONS INFORMATION
R
R
P
LOSS
1
1
I
L
||
=
=
R
R
2
( )
1
R
R
f
=
||
1
D
1
( )
R
=
(
L
DCR at
2
(
;
V
V
IN MAX
OUT
R
(
2
=
L
20
1–
R R
)
1
1
–
°
L
–
C C
V
R
, decreases with higher induc-
•
V
V
R
OUT
)
1
OUT
IN
D
•
D
1
)
•
V
OUT
IN
:
Accepting larger values of ΔI
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔI
ΔI
The inductor value also has secondary effects. The tran-
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
25% of the current limit determined by R
inductor values (higher ΔI
lower load currents, which can cause a dip in effi ciency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High effi ciency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Actual core loss is independent of core size for a
fi xed inductor value, but it is very dependent on inductance
value selected. As inductance increases, core losses go
down. Unfortunately, increased inductance requires more
turns of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
for high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates hard, which means that
inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase
in inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3890-1: one N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
L
occurs at the maximum input voltage.
L
L
) will cause this to occur at
=0.3(I
L
allows the use of low
LTC3890-1
MAX
). The maximum
SENSE
. Lower
17
38901f